Over the past century the demand for energy has grown exponentially. With the growing demand for energy, many different energy sources have been explored and developed. One of the primary sources for energy has been and continues to be the combustion of hydrocarbons. However, the combustion of hydrocarbons usually results in incomplete combustion and non-combustibles that contribute to smog and other pollutants in varying amounts.
As a result of the pollutants created by the combustion of hydrocarbons, the desire for cleaner energy sources has increased in more recent years. With the increased interest in cleaner energy sources, fuel cells have become more popular and sophisticated. Research and development on fuel cells has advanced to the point that many speculate that fuel cells will soon compete with the gas turbine for generating large amounts of electricity for cities, the internal combustion engine for powering automobiles, and batteries that are used for a variety of electronics applications.
Fuel cells conduct an electrochemical energy conversion of hydrogen and oxygen into electricity and heat. Fuel cells are similar to batteries, but they can be “recharged” while providing power.
Fuel cells provide a DC (direct current) voltage that can be used to power motors, lights, or any number of electrical appliances. There are several different types of fuel cells, each using different types of chemical reactions to produce electricity and heat. Fuel cells are usually classified by the type of electrolyte used. The fuel cell types are generally categorized into one of five groups; proton exchange membrane (PEM) fuel cells; alkaline fuel cells (AFC); phosphoric-acid fuel cells (PAFC): solid oxide fuel cells (SOFC); and molten carbonate fuel cells (MCFC).
The PEM fuel cells are currently believed to be the most promising fuel cell technology, and use one of the simplest reactions of any fuel cell. Referring to FIG. 1, a PEM fuel cell is illustrated at 10 which includes four basic elements: an anode 12; a cathode 14: an electrolyte (PEM) 16; and a catalyst 18 arranged on each side of the electrolyte 16.
Anode 12 is the negative post of the fuel cell and conducts electrons that are freed from hydrogen molecules such that the electrons can be used in an external circuit 20. Anode 12 includes channels 22 etched therein to disperse the hydrogen gas as evenly as possible over the surface of the catalyst 18.
Cathode 14 is the positive post of the fuel cell, and has channels 24 etched therein to evenly distribute oxygen (usually air) to the surface of the catalyst 18. Cathode 14 also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water. Water is the only by-product of the PEM fuel cell.
The electrolyte 16 is the proton exchange membrane (PEM) 16. The PEM is a specially treated porous material that conducts only positively charged ions. PEM 16 prevents the passage of electrons. In a working fuel cell, PEM 16 is sandwiched between anode 12 and cathode 14.
Catalyst 18 is typically a platinum powder thinly coated onto carbon paper or cloth. Catalyst 18 is usually rough and porous so as to maximize the surface area of the platinum that can be exposed to the hydrogen or oxygen. Catalyst 18 facilitates the reaction of oxygen and hydrogen.
The operation of the fuel cell can be described generally as follows. Pressurized hydrogen gas (H2) enters the fuel cell on the anode 12 side. When an H2 molecule comes into contact with the platinum on catalyst 18, the H2 molecule splits into two H+ ions and two electrons (e−). The electrons are conducted through the anode 12, where they make their way through external circuit 20 that may be providing power to do useful work (e.g., turning a motor or lighting a bulb 26) and return to the cathode 14 side of the fuel cell.
Meanwhile, on the cathode 14 side of the fuel cell, oxygen gas (O2) is being forced through the catalyst 18. In some PEM fuel cell systems, the O2 source can be air. As O2 is forced through catalyst 18, each O2 molecule forms two oxygen atoms, each having a strong negative charge. The negatively charged oxygen atoms attract the H+ ions through PEM 16 such that two H+ ions combine with an oxygen atom and two of the electrons from the external circuit to form a water molecule (H2O).
The PEM fuel cell reaction just described produces only about 0.7 volts, therefore, to raise the voltage to a more useful level, many separate fuel cells are often combined to form a fuel cell stack.
PEM fuel cells typically operate at fairly low temperatures (about 80° C./176° F.), which allows them to warm up quickly and to be housed in inexpensive containment structures because they do not need any special materials capable of withstanding the high temperatures normally associated with electricity production.
As discussed above, each of the fuel cells described uses oxygen and hydrogen to produce electricity. The oxygen required for a fuel cell is usually supplied by the air. In fact, for the PEM fuel cell, ordinary air is pumped into the cathode. However, hydrogen is not as readily available as oxygen.
Hydrogen can be difficult to generate and distribute. When hydrogen is generated from a fuel source such as sodium borohydride (NaBH4), the reactions generate waste products in the form of precipitated waste which includes foam or froth. Typically the hydrogen produced in a reactor is separated by a unit discrete from the reactor. The separation scheme is typically a gravity-aided disengagement chamber to break the foam generated in the reactor. With the use of a discrete separator, the flow through the reactor tends to become unstable and difficult to control due to the formation of the waste. The lack of control is exacerbated by the need for very low flow rates for hydrogen generation in portable power applications.
Batched processes for producing hydrogen which use sodium borohydride as the fuel source can also be difficult to control. With batched processes, the generation efficiency is low and only a discrete amount of hydrogen can be produced at a time. Furthermore, precipitated waste is generated from the hydrogen-producing reaction which collects in the reactor.
Hydrogen bubbles can be formed in the reactor which increases the backpressure of the fuel source. To obtain a constant hydrogen supply, the pressure of the fuel source must be constantly changed to overcome the backpressure caused by the hydrogen bubbles.
One approach used to control the generation of hydrogen gas is to add a catalyst in a controlled fashion to control the speed of the reaction. While this approach can improve the control problems caused by the formation of waste and hydrogen bubbles, this approach slows the reaction rate and decreases the amount of hydrogen produced. Also, additional equipment is required to carefully control the administration of the catalyst.
In view of the above, there is a need for an improved hydrogen gas generation approach which improves the efficiency and control of hydrogen gas generation.